1. Field of the Invention
The present invention relates to an optical disk apparatus, and more particularly, to a device for reproducing data from an optical disk on which synchronization information is formed in accordance with a specific rule.
2. Related Art
The optical disk apparatus hitherto optimizes recording power by means of techniques called OPC (Optimum Power Control) and ROPC (Running Optimum Power Control), thereby recording data. According to the OPC technique, test data are recorded in a predetermined area (PCA) of the optical disk while variously changing the recording power before recording data, and the test data are reproduced. Recording power at which the quality (e.g., a β value) of the reproduced signal becomes optimum is selected and taken as optimum recording power. In the mean time, according to the ROPC technique, consideration is given to the fact that the sensitivity of a recording film of the optical disk is not necessarily uniform within the plane. When data are actually recorded at the optimum recording power determined through OPC, the quantity of return light arising at the time of recording of data is monitored. The recording power is controlled by feedback such that the quantity of return light assumes a constant value. In general, the quantity of light of level B is used as the quantity of return light. The level B represents the quantity of return light acquired when a pit is formed upon exposure of the optical disk to recording power (i.e., the quantity of return light resulting from the recording power having been diffracted by the pit). Specifically, when the quantity of return light of level B is lower than a given value, pits are determined to be formed excessively. Hence, the recording power is reduced. In contrast, when the quantity of return light of level B is higher than the given value, pits are determined not to be formed sufficiently, and the recording power is increased.
Here, data having pit lengths 3T to 11T are recorded on a CD-R/RW or the like. However, difficulty is encountered in monitoring the quantity of return light arising at the time of recording of data having a short pit length to control the recording power through feedback. In light of this difficulty, the quantity of return light arising at the time of recording of data having the longest pit length 11T is detected, thereby effecting ROPC. The CD-R/RW complies with specifications of 11T being included in the synchronization information (SYNC) and of 11T continuously appearing twice at all times on a per-frame basis. Any of the two 11T pit lengths inevitably turns into a mark(i.e., an area where a pit is formed by radiation of the recording power). Therefore, ROPC is carried out cyclically (at the timing of synchronization information to be cyclically inserted) through use of the 11T pit length, thereby making an attempt to cyclically optimize the recording power.
In the case of a DVD-R/RW or the like, 3T to 11T pit lengths are formed as in the case of the CD-R or the like. However, a 14T pit length is used for synchronization information. In contrast with the CD-R, the DVD-R/RW complies with specifications of the 14T pit length being not continuously inserted twice (i.e., a mark and a space are not inserted as a pair) but being inserted only once in one frame (93 bytes). Taking the pit length 14T as a mark or a space is arbitrary. For example, when all of the pit lengths 14T are set as a space, pits are not formed to the longest pit length 14T, and hence ROPC cannot be conducted. Consequently, the recording power cannot be cyclically optimized, which in turn poses a problem of difficulty in maintaining recording quality. As a matter of course, ROPC can also be considered to be enabled by means of setting all of the 14T pit lengths as marks. However, when all of the 14T pit lengths are set to marks, a DSV value is increased, which may result in an increase in low-frequency components. Here, the DSV (Digital Sum Value) is a value determined by accumulating all bits in a sequence from the top while one status (e.g., 1) of the bit sequence—which takes two statuses—is taken as +1 and the other status (e.g., 0) of the same is taken as −1. The low-frequency component (a DC component) becomes small if the DSV value is small, as a result of which the recording and reproducing quality is improved.
A technique described in Japanese Patent Laid-Open Publication No. 2003-91819 is for setting marks and spaces as synchronization information such that they are alternately arranged. According to this technique, ROPC, or the like, is carried out in accordance with the synchronization information about the marks, whereby an attempt can be made to optimize the recording power.
As mentioned above, as a result of the marks and the spaces being alternately formed as synchronization information, ROPC or the like can be cyclically carried out, and the DSV value is also suppressed, thereby diminishing the low-frequency components of a reproduced signal. In addition, the synchronization information alternately appears according to a predetermined rule rather than arbitrarily appearing in the form of either marks or spaces as in the conventional technique, so that the feasibility of various reproduction processing can be expected.